For Edward Gibbon, the decline of the Roman Empire was a matter of blame. He did not hesitate to condemn a fatal combination of violent barbarian invasion and the growing popularity of Christianity with its pious preference for monasticism over militarism and its eagerness to welcome the end of the world. For Victorians, the fall of an imperial superstate was, above all, a matter of morality. Here was a lesson for all empire-builders: effete rulers intoxicated by the vain pomp of their glittering court ceremonies would inevitably be overwhelmed by their more vigorous and manly enemies. The Roman Empire had collapsed under the rotten weight of its own degeneracy. Many modern historians - sometimes smugly non-judgemental - have preferred to think instead in terms of the movement of peoples (rather than the invasion of barbarians) and the transformation (rather than the decline and fall) of the Roman Empire. The emergence of medieval Europe is not a matter of fault. No one needs to be held responsible.

It is to Chris Wickham's great credit that he has stepped boldly and decisively away from these old disputes. The groundwork was laid in his magisterial Framing the Early Middle Ages: Europe and the Mediterranean, 400-800 (Oxford University Press, 2005), which surveyed the complex social and economic changes that shaped the period. In The Inheritance of Rome, Wickham adds political and cultural history, but still resolutely refuses to offer any grand explanatory narrative. His central tactic is to decouple the Middle Ages from both the Roman Empire and early modern Europe. It is to be treated as a period in its own right: not as a long and tedious intermission stretching between the high summer of the classical world and its supposed rediscovery in the Renaissance. Wickham's aim is to write a history that is neither overshadowed by the break-up of the Roman Empire, nor driven by a concern to find the origins of European liberalism, democracy or the nation state.

In that sense, The Inheritance of Rome stands rather uncomfortably as the second volume in Penguin's projected eight-volume (three already published) History of Europe. This seems precisely the kind of old-fashioned enterprise that Wickham's approach is aimed at undercutting. Readers demanding a dynamic, forward-moving explanation of how the early Middle Ages helped in the making of Europe will be disappointed. Those who are willing to have their attention focused on the period itself - rather than worrying about the causes of imperial decline or seeking the early stirrings of modernity - will find instead a rewarding set of carefully constructed comparative studies spanning six centuries and a world that stretches from Córdoba to Cairo and from Offa's Dyke in Britain to the great libraries of Baghdad.

The Inheritance of Rome begins in the West with the establishment of new, post-Roman states in France, Spain, Italy, Britain and Ireland. It then turns eastwards. Here the surviving half of the Roman Empire, with its capital at Byzantium, continued to defend its hold on the eastern Mediterranean, with less success from the eighth century when it faced an aggressive and dynamic Arab state. The history of the 'Abbasid Caliphate is hardly ever included in conventional histories of Europe. It is one of the most rewarding pay-offs of this comparative project that Wickham places cultural and political developments in the Islamic world from AD 750 against the better known histories of Charlemagne in Francia (768-814) and Alfred the Great in England (871-899).

To understand the history of Europe it is necessary to maintain a Mediterranean-wide perspective (that expansive idea might be thought of as Wickham's own particular inheritance from Rome). Such a breadth of vision exposes important similarities: land remained the basis of aristocratic wealth, the extraction of income depended on the systematic exploitation of peasants, and the stability of medieval states was founded on the maintenance of a political community focused on a royal court. Within this framework there was substantial variation. Religion played a much greater role in the less deeply rooted kingdoms of western Europe than the wealthier and more confident Byzantine and Islamic worlds. Unlike Charlemagne, neither emperors nor caliphs sought legitimacy for their political programmes by seeking the explicit moral approval of clerics. The close coalition of church and state - so much part of the conventional image of the Middle Ages - turns out to have been more limited in its scope and impact. The East offers a striking contrast to the West.

Wickham's comparative project also challenges cherished national histories. The medieval world was remarkably international in its outlook. Alfred the Great's reformation of the English state owed much to his conscious adoption of policies successfully pursued by Charlemagne and his successors. Developments in an arc of kingdoms from Asturias-Léon in northern Spain through Ireland, Scotland, Scandinavia, the Slavic lands and into Rus reveal attempts by rulers to establish more elaborate political and military structures to underpin their rule. Again the variations are important. The establishment of centralised kingdoms was less successful in Wales and Ireland than in Rus, Bulgaria, Denmark and Asturias-Léon. But there is also a definite pattern: by AD 1000, Europe north of the Rhine-Danube (the old Roman imperial frontier) had crystallised into a set of recognisable states formed on the model of Francia or Byzantium. This is perhaps the most important political inheritance of Rome.

It is the steady accumulation of scholarly detail that allows Wickham to trace these complex developments across such a vast stretch of time and space. Rather than portray the past in broad, impressionistic terms, Wickham is master of a pointillist narrative style. 'I have tried consistently to stress the difference of local experience. I have compared rather than generalized.' The Inheritance of Rome is made up of a series of beautifully drawn miniatures presented without jaunty modern parallels and without the currently fashionable self-indulgence of trying to imagine 'what it was like to be there'. Rather, it is the differences from our own experiences that animate much of Wickham's understanding of the medieval Christian and Islamic worlds: these are sophisticated societies without liberalism, secularism or toleration, and which regarded social hierarchy, servility to superiors and the inferiority of women as normal and morally defensible.

Of the hundreds of medieval authors and thinkers that Wickham has read, he disarmingly confesses that he would enjoy meeting 'with any real pleasure' only the fifth-century Christian theologian Theodoret of Cyrrhus, Pope Gregory the Great and Einhard the ninth-century scholar, biographer of Charlemagne and adviser to Louis the Pious. Across six centuries just three individuals: a striking reminder that what matters to Wickham is that he should always keep his distance. The test of the historian is to capture the foreignness of the past without resorting to ridicule, disapproval or dislike. The challenge is to engage the interest of the reader without compromising the disorienting sense of the strangely unfamiliar. This is the outstanding achievement of The Inheritance of Rome. In a supremely humane and intelligent book Chris Wickham has presented medieval Europe in all its vivid richness and variety - without for a moment ever wanting to be there.

Christopher Kelly is a Fellow of Corpus Christi College, Cambridge. His latest book, 'Attila the Hun: Barbarian Terror and the Fall of Rome', was published by Bodley Head last year.